1) Field of the Invention
The present invention relates to electrochemical sensors intended for measuring the concentration of a chemical substance in an aqueous medium. Such devices find a particularly interesting but not exclusive application, to the detection of the amount of dissolved oxygen in water, particularly in basins of sewage works.
The invention more particularly relates to a method for self-calibrating an electrochemical sensor and to a method for self-cleaning this sensor. The invention also relates to particularly suitable electrodes for forming this sensor.
2) Description of Related Art
The electrochemical sensors of the aforementioned type necessarily include a working electrode, a reference electrode and a counter electrode. Another type of such sensors which further include a so-called generating electrode and its counter electrode, is also known. By adding both of these latter electrodes, the effect of which is to generate changes in the concentration of the species present in solution, it is possible to locally control the environment of the working electrode.
For example, the pH of the solution may be locally changed by applying a current to the generating electrode. A cathode current will cause the production of OH− ions (the pH then becoming more basic) and conversely, an anode current will cause the production of H+ ions (the pH then becoming more acidic). A counter electrode associated with the generating electrode, a counter electrode associated with the working electrode and a reference electrode are required for making a complete sensor.
It will easily be understood that it is particularly advantageous to use, as a working electrode, electrodes of very small dimensions, not only because this allows the gap between the generating electrode and the working electrode to be reduced, but also because the effects of the turbulence of the liquid at the cell are found to be minimized. Such electrodes of small dimensions are indifferently called subsequently in the description, “micro-electrodes” or “micro-discs”, this last name being due to the fact that the micro-electrodes are most often of a circular shape.
Document WO 02/095387 describes a sensor as mentioned earlier and illustrated in FIG. 1. It uses an electrically conducting substrate 10, advantageously made in doped silicon and the lower face of which is covered with a metallization layer 11. Its upper face is covered with a passivation layer 12 formed by stacking two sublayers of SiO2 and Si3N4, known to have excellent stability in an aqueous medium.
The passivation layer 12 is provided with circular apertures 13. Its upper face and the apertures 13 are covered with a conducting layer 14 bearing reference 16 when it is on the layer 12 and reference 18 when it forms a micro-disc lying on one of the circular apertures 13. The layer 14 is pierced with a network of annular apertures 19 each surrounding one of the micro-discs 18.
The layer 16 forms the generating electrode whereas all the micro-discs 18, electrically connected in parallel with each other, form together the working electrode of the system.
The electrochemical sensors find an interesting application in the measurement of the concentration of dissolved oxygen in basins of sewage works. Indeed, waste water is treated by means of bacteria which are very sensitive to this dissolved oxygen concentration. The information provided by the sensors provides a control on an aeration system of the basin so that the conditions are favorable to bacteria. However, the sensors present on the market do not give satisfactory results, notably because of various contaminations and organic materials which are deposited on the electrode, thereby changing its sensitivity. With conventional sensors, for controlling contamination, one generally resorts to mechanical methods of the pulsed air, pressurized water jet, abrasion type methods which all have a lot of drawbacks.
In order to remove contamination from the surface of the sensor, the application filed under number EP 04 405039.1 proposes the use of a diamond-based generating electrode. When a voltage is applied to it, this electrode generates strongly oxidizing species, such as hydroxyl radicals, capable of efficiently burning organic materials.
However, with the effect obtained by these oxidizing species, it is not possible to totally prevent any contamination. Over the course of time, the sensor is gradually affected, which deteriorates the accuracy of the measurement.